Polymers are susceptible to thermal oxidative degradation. This problem is acute during processing at elevated temperatures and/or high shear, but long term, post processing stabilization is also required for many polymeric articles. A variety of compounds and strategies to combat a variety of polymer degradation issues have been developed, and stabilizers such as phenolic antioxidants, hindered amine light stabilizers, ultraviolet light absorbers, organophosphites, antioxidants, metal salts of fatty acids, hydrotalcites, metal oxides, epoxidized oils, hydroxylamines, amine oxides, lactones, and thiosynergists are common commercial entities. In order stabilizers to be effective in the final composition or article, they must survive processing in a concentration sufficient for activity.
It must also be remembered that often a thermoplastic polymer resin will encounter more than one high temperature processing step before formation of the final article. For example, a resin may be extruded at high temperature to fully incorporate the stabilizers and then formed into particles, pellets, etc being further exposed to high temperatures during formation of the final article, e.g., during molding, film formation or fiber spinning. Decomposition and other loss of antioxidants during high temperature processing is common and consideration must often given to whether adequate stabilization has been provided to not only provide protection for all of the processing steps, but also to meet any post processing stabilization needs of the final article. Of course one could add antioxidants prior to high temperature step in a multi step manufacturing process, but this is not ideal. Further manufacturers may purchase polymer feedstocks that already contain thermal stabilizers and often would prefer to not add additional antioxidants during processing to form the final article.
Stabilization strategies for various polyolefin resins, e.g., polypropylene, polyethylene and olefin co-polymers, depend on the specific type of resin, for example, various polyethylene resins include HDPE, LDPE, LLDPE, etc., manufacturing process, gas-phase, slurry, solution etc., and catalyst Ziegler-Natta, Chromium, metallocene, etc., used in the polymer production. The end use of the resin also plays a role in selecting the appropriate stabilization protocol.
For example, the manufacture of certain polymeric articles, such as films and fibers, often require the use of severe processing conditions. Many articles may have other performance needs requiring specially formulated stabilizer packages. Certain molded articles, such as polyethylene pipes, are formed under demanding conditions and then subjected to extreme conditions placing high demands on the physical integrity of the polymer composition.
Organophosphites, often in combination with hindered phenols, are used broadly in the stabilization of polyolefins as non-discoloring antioxidants during melt processing, fabrication, and long term applications. Commonly used phosphites include tris-nonylphenyl phosphite (TNPP) and tris(2,4-di-t-butylphenyl)phosphite; Commonly used sterically hindered phenols include and 2,6-di-t-butyl-4-ethyl-phenol and 3,5-bis(1,1-dimethylethyl)-4-hydroxy-benzenepropanoic acid derivatives such as tetrakismethylene (3,5-di-t-butyl-4-hydroxylhydrocinnamate)methane and octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate.
TNPP and tris(2,4-di-t-butylphenyl)phosphite are commonly used in conjunction with, e.g., octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate for melt stabilization of polyethylene. However, combinations of many phosphites with octyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate exhibit poor gas fading and high gel content when incorporated in polyethylene resins, which render these stabilizers unsuitable for film applications.
U.S. Pat. No. 8,188,170, incorporated herein by reference, discloses that processing polyethylene in the presence of a combination of phosphite stabilizers and select phenols, e.g., 1,3,5-tris-(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)isocyanurate, 1,3,5-tris-(3,5-dicyclohexyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)isocyanurate, 1,3,5-tris(4-t-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-Triazine-2,4,6-(-1H,3H,5H)-trione, and 1,3,5-tris-(3,5-di-tert-butyl-4-hydroxybenzyl)-2,4,6-trimethylbenzene produces a polyethylene resin with low gel content and enhanced resistance to gas-fading well suited for film formation. Liquid phosphites are preferred in US 20100197837, as they are typically more readily processed and/or considered to be more compatible with the resin. In certain instances, better results are achieved with liquid phosphites.
U.S. Pat. No. 7,468,410 discloses a liquid phosphite stabilizer which is a mixture or tris-(mono-alkyl)phenyl phosphite esters. The liquid phosphite of U.S. Pat. No. 7,468,410 provides improved processing and gas-fading activity in linear low density polyethylene formulations vs the solid tris(2,4-di-t-butylphenyl)phosphite.
As mentioned above, the manufacture of polyolefin films presents specific challenges. These films include biaxially-oriented, blown polyolefin films used in the production of garbage bags, shopping bags, food wraps, and any number of articles requiring polymer chain orientation in both the machine direction (MD) and the transverse direction (TD) of the film. Both cast films and blown films are known having the requisite biaxial-orientation with good mechanical properties, namely tensile strength, elastic modulus, and impact resistance, in both the machine and transverse directions.
However, a polyethylene melt which has little long chain branching and narrow rheological breadth tends to form an unstable film bubble which can be described as bubble breathing (vertical movement of the neck as a function of time), dancing (circular rotation of the bubble around the axis formed by the die center), or other movement relative to the die which is random in nature. These instabilities cause poor gauge distribution (i.e., variations in film thickness) and may also result in process interruptions that will generally lead to a reduction of the quantity of acceptable film that is produced. This problem can be exacerbated by the presence of phenols and phosphites.
U.S. Pat. No. 6,930,137 overcomes this problem in blown polyethylene films by formulating with a lower amount of antioxidants than typically employed. i.e., less phenolic and, especially, less phosphite antioxidants. While this may aid in the formation of films with desired physical properties, the reduction in antioxidant level will mean that there is less stabilizer to protect the film during use.
U.S. Pat. No. 6,022,916 details some of the problems associated with forming films via melt extrusion, for example, complications arising from incompatible materials, such as filters clogged by solid materials, more common with solid phosphites such as tris(2,4-di-t-butylphenyl)phosphite. Issues related to exudation of formulation components and processing byproducts can also force the shutdown of productions lines for cleaning.
U.S. Pat. No. 4,600,633, incorporated herein by reference, and EP 0278569 discuss methods for preparing cast films, e.g., HDPE cast films. Such films can be prepared by stretching a gel sheet, i.e., a composition comprising polymer and a solvent, at elevated temperatures and temperatures of 250° C. and higher are often encountered. Cast films are also prepared by high temperature extrusion, typically followed by rolling on one or more cooling rolls, and processing temperatures of 280° C. may be encountered.
HDPE pipe can be solid wall or a layered structure such as profile pipe which has a thinner inner lining and an exterior shell. Pipes are typically prepared using an extrusion process; fittings pipes are often prepared injection molding. In use conditions for many pipe applications require strength and non-leaching characteristics among other performance criteria requiring care in stabilizer selection.
It is well known in the art that antioxidant action of phosphite stabilizers leads to decomposition of the phosphite. However, the first formed decomposition products often have antioxidant activity, but again, the antioxidant activity of these decomposition products is also associated with further decomposition of stabilizer, and between decomposition and hydrolysis, the amount of active phosphite additive in the polymer is ultimately exhausted. Nonetheless, when considering the long term effectiveness of a phosphite stabilizer, one should take into account both the amount of original phosphite species that remains as well as the amount of the phosphorus containing byproducts that also have antioxidant activity. The amount of phosphorus present in a composition in any species having antioxidant activity, whether the originally added phosphite or other active decomposition products thereof, can be referred to as active phosphorus.
U.S. Pat. No. 7,888,414, incorporated herein by reference, provides a liquid phosphite stabilizer, useful as a processing stabilizer for polyolefins, which consists essentially of a mixture of phosphite compounds that are generally solid when individually present at room temperature. For example, certain mixtures of tris 4-tert-butyl phenyl phosphite, tris 2,4-di-tert-butyl phenyl phosphite, bis(4-tert-butylphenyl)-2,4-di-tert-butylphenyl phosphite, and bis(2,4-di-tert-butylphenyl)-4-tert-butylphenyl phosphite are shown to be liquid at room temperature as are certain mixtures of tris 4-tert-pentyl phenyl phosphite, tris 2,4-di-tert-pentyl phenyl phosphite, bis(4-tert-pentylphenyl)-2,4-di-tert-pentylphenyl phosphite, and bis(2,4-di-tert-pentylphenyl)-4-tert-pentylphenyl phosphite.
It has been found that liquid phosphites according to U.S. Pat. No. 7,888,414, and related U.S. Pat. Nos. 8,008,383; 8,008,384, and 8,178,005 have surprising benefits relative to other phosphites, including other liquid phosphites, in polyolefin compositions useful in fiber and film formation and other demanding applications such as pipe. The liquid phosphite often contains an additive, such as a hydroxyalkylamine, to prevent hydrolysis as in U.S. Pat. No. 8,048,946. Surprisingly, these liquid phosphites are as active, or more active, than other phosphites in polyolefin compositions during, e.g., high temperature extrusion or long term use, even while retaining a higher amount of retained active phosphorus.